Dicamba is a selective herbicide currently used for treating e.g. corn, wheat or grassland. It kills broadleaf weeds before and after they sprout. The trivial name dicamba refers to the compound 3,6-dichloro-2-methoxybenzoic acid. The estimated global demand for dicamba in 2012 was about 12.000 metric tons per year. However, it is expected that the global demand for dicamba will increase significantly.
Dicamba is typically produced on an industrial scale from 2,5-dichlorophenol using carboxylation under Kolbe-Schmitt conditions, methylation and subsequently saponification/acidification. 2,5-Dichorophenol in turn can be obtained from 1,4-dichlorobenzene or 1,2,4-trichlorobenzene. The synthetic route via 1,4-dichlorobenzene involves nitration and subsequent diazotation, and, therefore is undesired for use on an industrial scale. The synthetic route via 1,2,4-trichlorobenzene suffers from limited availability of this starting material and from the formation of several byproducts which are formed in the synthesis of 2,5-dichlorophenol.
In order to meet the increasing market demand for compounds such as dicamba, there is a need in the art for processes providing improved yield and/or regioselectivity in the hydrolyzation of 1,2,4-trihalobenzene, such as 1,2,4-trichlorobenzene, so that the limited resources of these compounds can be used more efficiently.
In view of the above, there is further a need in the art for a process for obtaining 2,5-dihalogen substituted phenols, such as 2,5-dichlorophenol, with improved yield and/or regioselectivity. Moreover, there is a particular need in the art for processes and reaction sequences for obtaining dihalogen substituted salicylic acid derivatives, especially including dicamba, in improved yields.
A further object of the present invention is to provide reaction conditions allowing an improved regioselectivity and/or yield in the hydrolyzation of 1,2,4-trihalobenzene for obtaining dihalogen substituted phenols or ultimately dihalogen substituted salicylic acid derivatives, including dicamba. Another object of the present invention is the provision of an improved process for providing 2,5-dichlorophenol. It is a further object of the present invention to implement the improved process for the synthesis of dicamba on an industrial scale. It is another object of the present invention to obtain 2,5-dihalophenol in improved yield. Moreover, since 2,5-dihalophenol alkyl ether can easily be transferred to the corresponding 2,5-dihalophenol, it is a further or alternative object of the present invention to obtain an increased 2,5-regioselectivity.
The object of the present invention is to meet the above needs. In this context it should be noted that even minor improvements in the yield and/or 2,5-regioselectivity in reaction sequences for obtaining dicamba would provide a tremendous benefit. For example, an improvement of yield and/or 2,5-regioselectivity of 1% would provide an additional annual amount 120 metric tons of dicamba.